The objective is to investigate the hypothesis published by Billington in 1964, that sensitization of the mother during pregnancy against paternal antigens leads to nonpathological placenta hypertrophy and increased birthweight in succeeding pregnancies. Arrangements for a laboratory study have been active and 44 maternal sera have been tested against paternal leucocytes. Analysis of a large sample of study data has given convincing support to the hypothesis. Logistic complications which cause data return for effort have stopped collection of new cases. The accumulated data will soon be reviewed for publishability.